1. Field of the Invention
The present invention relates generally to processing titanium alloy workpieces and, more particularly, to a process of preventing surface discoloration of workpieces formed of titanium and its alloys during their ion implantation.
2. The Prior Art
Titanium-based alloys have come to the fore of late in replacing cobalt-based alloys that had been traditionally used as orthopaedic surgical implants. A number of reasons are responsible for the switch to titanium-based alloys. These include: excellent tensile strength, high fatigue strength, low density, high corrosion resistance, substantial ductility, a low modulus of elasticity compatible with bone structure that facilitates good adhesion thereto and, most importantly, excellent biocompatibility. The only questionable property of titanium-based alloys has proven to be their wear resistance. The poor wear performance of surgical implants made from titanium-based alloys has however been effectively improved upon by ion implantation, in particular by implantation of carbon and nitrogen ions directly into the surface of the surgical implants. See "Ion Beam Modification of Materials for Industry", Thin Solid Films, 118 (1984) 61-71; "The Wear Behavior of Nitrogen-Implanted Metals," Metallurgical Transactions, A 15 (1984), 2221-2229; and "Wear improvement of surgical titanium alloys by ion implantation"; J. Vac. Sci. Tech.A3 (6) Nov./Dec. 1985, 2670-2674.
While effectively improving the wear performance of titanium-alloy surgical implants, ion implantation of these implants causes the surfaces of the implants to discolor at spots. Such discoloration resembles tarnishing and exhibits a goldish-yellow or bluish color. When viewed by others, in particular by surgeons who perform the operations and by patients slated to receive such orthopedic implants, the sights of these discolored implants make their acceptance less than desirable, if not outright objectionable.